Not applicable.
1. Field of the Invention
The present invention generally relates to optical flow meters for measuring flow characteristics of a fluid in a pipe. More particularly, the present invention relates to an optical flow meter in which at least part of the optical flow meter mounts within the pressurized environment of the pipe. Still more particularly, the present invention relates to optical systems that are used to focus and collect light beams inside a pressurized pipe, and which are subjected to the fluid within the pipe.
2. Background of the Invention
In pipeline operations and other industrial applications, flow meters are used to measure the flow rate of gases or liquids moving through the pipeline or tubing system. In natural gas pipelines, for example, these flow rate measurements are used for custody transfer, leak detection, control, or for other purposes. Custodial transfers of hydrocarbons and other fluids require very accurate measurements of the fluid being transferred. The flow meter typically is used as the transfer point for a custody transfer to determine the amount of fluid that has been transferred. By accurately measuring the flow rate for a particular transfer period, the volume of liquid that flows through the pipeline can be determined, and a custody transfer volume ticket can then be prepared. The pipeline transportation fee is based on the volume of product moved through the system, i.e. the custody transfer volume. Thus, a custody transfer metering system is commonly referred to in the pipeline industry as the xe2x80x9ccash register,xe2x80x9d and pipeline operators take great care to maintain its measurement accuracy.
Various meters have been used to measure the flow of liquids in pipelines and other tubing systems. Existing meters can be separated into three general categories for purposes of discussion. The first type of meters is obstruction meters. Obstruction meters determine fluid flow rates by introducing a physical obstruction directly into the flow of fluid, and measuring the influence of the fluid on the obstruction. One type of obstruction meter is those that introduce a flow restriction in the pipeline, such as a reduced-diameter orifice plate. The flow restriction creates a pressure drop, which can be measured and used to calculate the fluid flow rate through the restriction. In addition to orifice meters, venturi meters and critical flow nozzles also make use of flow restrictions to measure fluid flow rates. Other obstruction meters include vortex meters.
Obstruction meters have the inherent disadvantage of extracting energy from the fluid flow as a consequence of the pressure drop. The inefficiencies caused by the physical obstruction require additional pumping capacity to overcome. In addition, the physical obstruction also acts as an impediment to using pipeline pigs or other devices that scrub the interior of the pipeline. In addition, because the obstruction-type meter is placed directly in the fluid flow, the meter components may be subject to premature wear, requiring maintenance and replacement. These meters also are characterized by low turndown ratios (i.e. a limited range of flow rates for which accurate measurements are possible) due to the non-linear relationship between flow rate and pressure drop.
The second type of flow meter is kinematic meters. Kinematic meters determine the flow rate by directly sensing the actual velocity of the fluid using a turbine blade assembly that rotates kinematically with the flow. The rotational speed of the turbine is measured using a frequency transducer, and then is empirically related to flow rate using an experimentally determined coefficient. Kinematic meters provide an output that is approximately proportional to volumetric flow rate and substantially independent of density. The primary disadvantage of kinematic meters is the presence of moving parts, the obstruction to the fluid flow, the requirement of electrical power, and the physical size of the meter.
The third class of flow meters are those that use non-intrusive methods to determine flow rate. The ultrasonic meter is the only meter in this category that has been commercially developed for use in high pressure natural gas pipelines. An ultrasonic meter compares the upstream and downstream transit times of an acoustic pulse from transducers located near the inside surface of the pipe. The fluid flow therefore is unrestricted, and thus these meters do not produce any significant pressure drop. These devices, however, require a relatively long installation length (on the order of a few pipe diameters), are limited to larger pipe sizes, and can suffer from acoustic noise in the vicinity of the meter.
As noted in U.S. application Ser. No. 09/065,364, entitled xe2x80x9cOptical Flow Meter Integrally Mounted to a Rigid Plate With Direct Optical Access to the Interior of a Pipe,xe2x80x9d incorporated by reference herein, optical flow meters are non-intrusive flow meters that potentially overcome many of the disadvantages that exist with existing commercial flow meters, if the optical flow meter can be designed in a sufficiently compact space, with adequate access to the pipe interior.
Flow rates may be determined with optical meters by measuring the velocity of particles suspended in a representative field in the fluid flow. The particles preferably are under 100 microns in diameter, and more preferably under 25 microns in diameter. The velocity is measured by determining the time-of-flight of these particles as they move between two discrete regions that have been illuminated with light from a laser. See e.g. D. H. Thomson, xe2x80x9cA Tracer Particle Fluid Velocity Meter Incorporating a Laser,xe2x80x9d Jour. Of Sci. Inst. (J. Phys. E.) Series 2, Vol. 1, 929-932 (1968). The time-of-flight concept has been applied to measure the air speed of an aircraft, as described in U.S. Pat. Nos. 4,887,213, 5,046,840 and 5,313,263.
Optical techniques also have been used by numerous investigators to make measurements in laboratory environments, such as wind-tunnels and turbo machinery. See R. Schodl, xe2x80x9cA Laser-Two-Focuse (L2F) Velocimeter for Automatic Flow Vector Measurement in Rotating Components of Turbomachines,xe2x80x9d Transaction of the ASME, Vol. 102, p. 412 (December 1980). Additionally, UK Patent 2,295,670 describes a configuration in which laser light from an argon ion laser is split by a Rochon prism, made parallel by a lens and then focused into two spots. Scattered light produced by particles passing through the two spots is imaged onto two photoelectric converters. Velocity is determined on the basis of the transit time of the particles passing between the two spots. U.S. Pat. No. 4,125,778 discloses a similar device, except that the relative position of the two spots can be rotated using an optical component. A variation on the time-of-flight principle using individually addressable laser diode arrays was described in U.S. Pat. No. 5,701,172 to measure the velocity profile inside a high pressure natural gas pipeline through a glass window. The image of the diode array produces a series of spots in space and the light scattered by small particles is collected and converted to electrical signals. The frequency content of the signal and the spacing of the spots of light are used to determine the flow velocity. Alternatively, the patent describes the system in combination with a holographic lens placed as a window in the pipeline wall, thereby permitting multiple measurement locations along one pipe diameter. However, the requirement of a relatively large glass window in the pipeline renders this design impractical for broad commercial applications because of cost and safety concerns.
Related U.S. application Ser. No. 09/065,364, entitled xe2x80x9cOptical Flow Meter Integrally Mounted to a Rigid Plate With Direct Optical Access to the Interior of a Pipe,xe2x80x9d describes an optical flow meter in which the optical components are mounted in a rigid plate that is placed within the pressurized pipe environment. According to the preferred embodiment, the plate fits within a conventional pressurized orifice meter in place of a convention plate carrier. This enables the optical components to have direct access to the fluid flow, without requiring use of a glass window in the pipeline. Because the optical components are mounted in the wall of the orifice plate, the meter does not produce any significant pressure drop in the passing fluid.
Thus, in the field of optical flow metering, there is a need to focus light to a point in space within a pressurized pipe and to collect the light scattered by particles in the fluid. The signal generated by one or more photo detectors then can be used to determine the velocity of the fluid in several ways, including Laser Doppler Anemometry (LDA) and time of flight anemometry. The optical meter typically is calibrated in air under laboratory conditions, prior to being used in a natural gas, or other measuring environment. The problem, however, is that the refractive index of air differs from the refractive index of the fluid being measured, thus compromising the integrity of the subsequent measurement when the meter is placed in the field. Put simply, the characteristics of the multiple light paths, which are used for the flow measurement, will vary depending on the medium in which the optics are placed. If the optical system measures velocity based on the transit times of particles between two beams, and the beams are displaced because of a differing refraction index, then the velocity measurement will be compromised. Additionally the performance of optics used to collect scattered light can be adversely affected. In particular, applications in pressurized gases pose significant difficulties, because the refractive index of the gas changes with pressure, temperature, and composition. Natural gas, for example, has an index of refraction that may vary from approximately 1.00 at atmospheric conditions to 1.04 at pressures encountered in high pressure pipelines. Such a variation in the index of refraction can cause significant degradation in the performance of conventional optical systems, to the point that they may no longer be functional.
One technique for overcoming the difficulties with refractive index variation were disclosed in U.S. patent application Ser. No. 09/065,364. In that disclosure, the use of quarter pitch gradient index (GRIN) lenses for collimation was suggested. For optical components in which larger apertures and faster (low f numbers) lenses are needed, a novel lens design was disclosed. This lens included (i) two or more elements that had very large differences in refractive index, such as CaF2 and ZnS; (ii) first and last surfaces that were concave and defined by spherical surfaces centered at the object and image locations, respectively; and (iii) cemented internal surfaces between the elements shaped in such a way as to provide all the focusing power necessary to image the object at the desired location. The use of spherical surfaces centered at the object and image locations ensures that the light enters and exits the lens perpendicular to the local surface, and therefore is not affected by the index of refraction of the fluid in contact with the lens faces.
While the optical meter disclosed in U.S. patent application Ser. No. 09/065,364 overcomes many of the problems with prior optical flow meters, the preferred embodiment disclosed therein requires multiple optical components that are subject to premature wear and contamination, as particles in the fluid may become entrapped between optical components. The image and object locations are fixed, and thus so is the light path used to measure flow rate. If the fluid contains particles, a significant portion of the light may be blocked by contaminants on optical surfaces in line with object or image locations. For example, if the end of an optical fiber is either the light source to be focused to a point in space, or the image location for light collected from a point in space, a small amount of dirt on the end surface of the fiber may very seriously degrade the performance of the system.
An alternative design that has been used in industry is a probe in which the optics are contained within a sealed package with low pressure gas (such as air) between the optical components and the window. Examples of probe designs may be found in U.S. Pat. Nos. 5,365,326 and 4,978,863. In addition, TSI manufactures a probe that includes optical components mounted in a sealed probe, with a window that permits light beams to be transmitted and received by the optical components inside the probe. The use of the sealed housing in these designs makes it possible to keep most of the optical surfaces clean. These probes generally have flat optical surfaces in contact with the fluid and hence are sensitive to the index of refraction of the surrounding fluid. This design, however, has some severe limitations, especially in the context of high pressure gases. When such an assembly is left in a high pressure gas environment, it is extremely difficult to prevent leakage of the gas into the sealed housing. Unless extreme precautions are taken the gas will tend to leak into the unit through the seals, either due to defects in the sealing materials, or through molecular diffusion. This leakage may degrade optical performance due to the differing refractive index of the air and the gas. An even more critical problem, however, may result when the probe is removed from the high pressure gas for inspection, servicing or replacement. Once the probe is removed, the internal pressure significantly exceeds atmospheric pressure, and the pressurized gas in the internal volume of the probe in effect makes the assembly a bomb that could explode at any time, thereby posing a significant risk to workers. Techniques to minimize this problem would be to fill the probe with a liquid or gel that would minimize gas leakage into the chamber. Such a design, however, raises problems in terms of gas solubility in the gel or liquid, and problems with optical performance due to a reduced difference in refractive index between the lenses and the cavities within the assembly.
It would be desirable if an optical flow meter could be designed that significantly reduces the problems caused by subjecting the meter to different flow mediums with a different index of refraction. It would also be desirable to develop an optical design that can be immersed in the fluid being measured, without creating unsafe pressure build-ups. In addition, it would be desirable if the optical components could be integrated in a package that would significantly reduce the problems caused by contaminants within the optical system. Despite the apparent advantages of such a system, to date no such system has been developed.
The present invention solves the deficiencies of the prior art by incorporating a very compact optical system design in an optical flow meter. The compact optical system design avoids problems with contamination of the image or light source location by integrating the optical delivery system into a single integrated package, with no internal cavities. According to the preferred embodiment, one or more optical fibers is attached to the optical system to avoid interference by particles in the fluid being measured. The optical design preferably comprises a pentaprism design that includes two internal mirrored surfaces. According to one embodiment, a concave spherical surface is used as the surface closest to the image. The concave spherical surface is centered at the image location to ensure that the light enters and exits the lens perpendicular to the local surface, and therefore is not affected by the index of refraction of the fluid in contact with the lens faces.
According to the preferred embodiment, the present invention relates to an optical flow meter that generates a signal containing information that may be used to describe fluid flow within a pipeline, based on the motion of particles suspended in the fluid. The flow meter preferably includes a narrow frequency light source and one or more fiber optic cables that transmit light from the light source to an optical delivery system disposed within the pressurized environment of the pipe. The optical delivery system preferably collimates and focuses two light beams into two parallel planes within the fluid flow. Alternatively, a Laser Doppler Velocimetry (LDV) optical system may be used to focus multiple light beams to a point within the pipeline. Light scattered by particles in the fluid is collected by an optical collector, which collects the light and transfers the light via one or more fiber optic cables to a light detector. The light detector converts the light pulses into electrical signals that are analyzed by a computer or signal processing device.
According to one embodiment of the present invention, a pentaprism design may be used as either the optical delivery system, or as the collector, or as both the optical delivery system and collector simultaneously, in an optical flow meter. The collector pentaprism design includes a concave surface that receives the scattered light from the image. The concave surface is designed as a spherical surface centered at the image. Consequently, light passes from the image perpendicularly through the concave spherical surface. The light passing through the concave spherical surface reflects off a mirrored surface that preferably has an elliptical shape. Light from the elliptical mirrored surface reflects off a flat mirrored surface that focuses light into a fiber optic cable that is firmly attached to base of the pentaprism.
According to another embodiment of the invention, the optical delivery system pentaprism design has a generally flat base surface, to which a fiber optic cable is firmly attached. The fiber optic cable carries light from the light source. Light from the end of the fiber optic cable reflects off a flat mirrored surface located opposite the flat base surface. Light from the flat mirrored surface strikes a side wall of the prism. This wall may be configured as an elliptical mirror, but preferably comprises a diffractive lens element. Light reflects off the diffractive element and is focused on the image. Between the diffractive lens element and image is the opposite side wall, which may comprise a concave spherical exit surface. To produce a pair of light beams, two pentaprisms may be effectively stacked together in a single integrated package. This may be done by stacking two diffractive elements on the side wall of the pentaprism to insure the two light beams are in parallel.
Additional details of the present invention will become apparent by studying the following Detailed Description of the Preferred Embodiment.